1. Field of the Invention
The invention relates to a process for preparing noble metal nanoparticles by reducing metal nanoparticles and controlling their particle size with surfactant, characterized in that the nanoparticles thus prepared can be dispersed in both polar and non-polar solvents and hence can be applied extensively in the electroless plating process used in the fabrication of electronic circuits as well as many catalytic reaction using catalysts such as platinum, palladium and the like.
2. Description of the Related Prior Art
In past decades, compared with conventional metal microparticles, metal nanoparticles exhibit excellent qualities such as increased surface-to-volume ratio, elevated surface energy, increased ductility after pressure loading, higher hardness, larger specific heat and the like (C. Suryanarayana, F. H. Froes, Metallugial Transactions A 1992, 23, 1071). Therefore, the applicability of metal nanoparticles receive extensively interest by the material-directed industry and science. For example, catalysts based primarily on a variety of metal nanoparticle are increasingly used in various catalytic reactions among which an advantage of high conversion on the catalytic reaction of unsaturated hydrocarbon is noted especially (L. N. Lewis, Chem. Rev. 1993, 93, 2693).
It is known that different particle size of the nanoparticle could result in different electronic energy band configuration, a size-dependent property, which in turn might have effects on the physical and chemical properties thereof. This property is very important for their industrial application such as, for example, in the electroless nickel plating using platinum (Pt) as the catalyst, wherein different electroless plating rates might vary with the particle size, as shown in FIG. 1 (Hamilton, J. F.; Baetzold R. C., Science, 1979, 205, 1213). Accordingly, it is the current topic in the industry as how to provide a process for the preparation and particle size distribution controlling of metal nanoparticles having various particle sizes.
Processes currently used for preparing metal nanoparticle are classified generally into chemical synthetic method and physical synthetic method. The chemical reduction method is widely used due to its high production capacity, simple preparation process, and low cost. In general, chemical reduction method comprises of dissolving inorganic metal salts in a solution containing polymer or surfactant dissolved therein, and then reducing said inorganic metal salts into metal nanoparticles with added reducing agents such as hydrazide, NaBH4, ethanol, hydrogen and the like.
The reducing agent added could have an effect on the particle size. An excess addition may result in a strong reducing power and hence too rapid the nucleation rate such that the resulting metal nanoparticles will be too large and have a non-uniform particle size distribution. On the contrary, too less addition of the reducing agent will lead frequently to a too low reducing rate such that the synthesis of the particle might take several days, and even may make the reduction of the inorganic metal salts impossible. Accordingly, the type and dose of the added reducing agent, as well as the mode of agitation are the critical parameters that increase the complexity of the production process.
U.S. Pat. No. 4,593,016 disclosed a process for preparing nanometer colloidal particles of tin-palladium alloy by dissolving palladium chloride and stannous chloride in HCl solution, respectively, mixing these two solutions and raising the temperature to 100xc2x0 C. such that the stannous ion could reduce the palladium ion into tin-palladium alloy nanoparticle. U.S. Pat. No. 5,187,209 disclosed the synthesis of Pt nanoparticles of about 3 nm by dissolving 5 ml 1.1xc3x9710xe2x88x922 mol/l Na2PtCl4 in 50 ml of a mixture consisting of equal portions of water and ethanol and containing 1.1 g copolymer having hydrazide functional group, and irradiating the resulting solution with a 500 W high pressure mercury lamp for 2 hours to reduce Pt into Pt nanoparticles.
U.S. Pat. No. 5,147,841 disclosed the preparation of Pt or Pd nanoparticles having particle sizes of 10-20 nm by a process comprising dissolving NacPtCl4 or NaPdCl6 in a n-octane solution of a cationic surfactant, didodecyldimethyl-ammonium bromide (DDAB), and adding NaBH4 or hydrazide to reduce the Pt2+ or Pd2+ ions in the solution.
U.S. Pat. No. 5,759,230 disclosed the synthesis of 10 nm Pd nanoparticles and 40 nm Ag nanoparticles by dissolving PdCl2 or AgNO3 in ethanol used also as the reducing agent and heating at 120-200xc2x0 C. under reflux for 1-3 hours.
U.S. Pat. No. 5,332,646 disclosed a process comprising reducing metallo-organic salt of Pd and Pt that have been dissolved in a solution of surfactant/organic solvent and hence synthesizing an effectively dispersed Pd and Pt metal nanoparticle of 2-50 nm.
U.S. Pat. No. 5,620,584 disclosed the synthesis of Pd nanoparticles having various particle sizes by using 90 ml 0.1 M tetraoctylammonium bromide solution in THF as the electrolytic solution and tow pieces of palladium sheet as the cathode and anode, and synthesizing Pt nanoparticles with varying electric current density via an electrochemical reaction as oxidation of ions by the anode and reduction by the cathode.
U.S. Pat. No. 6,103,868 disclosed a process for preparing gold nanoparticles by dissolving 150 mg HAuCl4.3H2O in 25 ml de-ionized water into a yellow aqueous solution that was mixed with a solution of 0.365 g N(C8H17)4Br/25 ml toluene under intensively stirring, while adding 0.151 g NaBH4 to yield reduced gold nanoparticles.
In summary, as used in conventional techniques, surfactants have been used as the protective agent for particles due to the fact that the surfactant dissolved in a solvent can form micelle. Further, if the metal nanoparticles were synthesized in an aqueous solution containing an aqueous phase surfactant, since a surfactant has a feature that a surfactant molecule in an aqueous solution will have its hydrophilic group face outwardly, and its lipophilic group face inwardly, the particles can be re-dispersed only by the aqueous phase, while the particles will be un-stable in the organic solvent such that it will render the precipitate not effectively dispersed. Likewise, in case of synthesizing metal nanoparticles in an organic solvent, due to the fact that the surfactant molecule will form a structure of reverse micelle in an organic solvent where its lipophilic group will face outwardly while its hydrophilic group inwardly, they can be dispersed stably only in an organic solvent.
Accordingly, there is a need in the art to provide a process for preparing metal nanoparticles that can be dispersed in both polar solvent and non-polar solvent.
The main object of the invention is to provide a process for preparing noble metal nanoparticles, characterized in that it comprises uses surfactant containing carboxylic group (COOxe2x88x92), sulfate group (SO42xe2x88x92) or sulfonate group (SO32xe2x88x92) to play the role of reducing agent, and that synthesis of metal nanoparticles having different particle size can be controlled by adjusting the carbon chain length of the surfactant or the synthesizing time.